vfio-user: add coalesced posted writes

vfio-user: add coalesced posted writes

Add new message to send multiple writes to server in a single message.
Prevents the outgoing queue from overflowing when a long latency
operation is followed by a series of posted writes.

Originally-by: John Johnson <john.g.johnson@oracle.com>
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-18-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>